This invention relates to a method and a circuit arrangement for monitoring the function of a symmetrical sensor bridge circuit having sensor elements, which are connected in a full bridge circuit. A semiconductor sensor in the form of a symmetrical bridge circuit, known from the prior art, and a differential amplifier circuit, which generates an output signal of the sensor bridge circuit, are often used for detecting a physical quantity, for example pressure, velocity, acceleration or the like. Within a defined measuring voltage range, this output signal from the sensor bridge circuit indicates the physical quantity to be measured, for example the pressure, as a mostly linear function.
When employing such a sensor bridge circuit in a safety-related measuring device, for example in the anti-blocking system of a motor vehicle, it is very important for the safety of the motor vehicle and its occupants that the function of the sensor be continuously monitored during its operation, and that, in case of a malfunction of the sensor, it is possible to immediately generate an alarm signal which indicates this malfunction. In this connection it is important that not only disruptions in the lines leading to and from the sensor bridge circuit, the supply lines and similar can be detected, but also malfunctions which can cause the outage of the pressure sensor, so that slowly occurring changes, for example changes of the characteristic curve of the pressure sensor, the amount of amplification of the integrated amplifiers, etc., which degrade the accuracy of the measurement, can be detected.
A known sensor error detection device for detecting a line break or a short circuit, known from U.S. Pat. No. 4,845,435, has a resistor for generating a sensor compensation output signal between an output connection on the ground side of a sensor and the sensor. A comparison of the sensor output signal generated in this way with a reference voltage makes possible the drawing of a conclusion regarding the status or the functional ability of the sensor by means of the output state of a comparator employed for this. If, for example, a sensor is connected with an A/D-converter, the dependability of the known arrangement for the detection of interruptions of the respective voltage supply connections, the output connections and the ground connections of the sensors is not assured.